The present invention relates generally to AC induction motor drives and, more particularly, to a method and apparatus for monitoring motor status using induced motor voltage.
Induction motors have broad application in industry, particularly when large horsepower is needed. In a three-phase induction motor, three phase alternating voltages are impressed across three separate motor stator windings and cause three phase currents therein. Because of inductances, the three currents typically lag the voltages by some phase angle. The three currents produce a rotating magnetic stator field. A rotor contained within the stator field experiences an induced current (hence the term “induction”) that generates a rotor field. The rotor field typically lags the stator field by some phase angle. The rotor field is attracted to the rotating stator field and the interaction between the two fields causes the rotor to rotate.
A common rotor design includes a “squirrel cage winding” in which axial conductive bars are connected at either end by shorting rings to form a generally cylindrical structure. The flux of the stator field cutting across the conductive bars induces cyclic current flows through the bars and across the shorting rings. The cyclic current flows in turn produce the rotor field. The use of this induced current to generate the rotor field eliminates the need for slip rings or brushes to provide power to the rotor, making the design relatively maintenance free.
To a first approximation, the torque and speed of an induction motor may be controlled by changing the frequency of the driving voltage, and thus, the angular rate of the rotating stator field. Generally, for a given torque, increasing the stator field rate will increase the speed of the rotor, which follows the stator field. Alternatively, for a given rotor speed, increasing the frequency of the stator field will increase the torque by increasing the slip (i.e., the difference in speed between the rotor and the stator fields). An increase in slip increases the rate at which flux lines are cut by the rotor, increasing the rotor generated field and thus the force or torque between the rotor and stator fields.
Referring to FIG. 1, a rotating phasor 10 corresponding to a stator magneto motive force (mmf) generally has an angle, α, with respect to the phasor of rotor flux 12. The torque generated by the motor is proportional to the magnitudes of these phasors 10, 12 but is also a function of the angle, α. Maximum torque is produced when the phasors 10, 12 are at right angles to each other, whereas zero torque is produced if the phasors 10, 12 are aligned. The stator mmf phasor 12 may therefore be usefully decomposed into a torque producing component 14 perpendicular to rotor flux phasor 12 and a flux component 16 parallel to rotor flux phasor 12.
These two components 14, 16 of the stator mmf are proportional, respectively, to two stator current components: iq, a torque producing current, and id, a flux producing current, which may be represented by quadrature or orthogonal vectors in a rotating or synchronous frame of reference (i.e., a reference frame that rotates along with the stator flux vector) and each vector iq and id is characterized by slowly varying DC magnitude.
Accordingly, in controlling an induction motor, it is generally desired to control not only the frequency of the applied voltage, hence the speed of the rotation of the stator flux phasor 10, but also the phase of the applied voltage relative to the current flow, hence the division of the currents through the stator windings into the iq and id components. Control strategies that attempt to independently control current components iq and id are generally referred to as field oriented control strategies (FOC).
There are many instances in which it is desirable to measure one or more parameters of motor operation. Typical parameters of interest include rotor speed, rotor direction, back EMF magnitude, and back EMF phase angle. During normal motor operation, adequate assumptions about these parameters can often be made based on the control that is implemented. For example, if a particular speed is commanded in an open loop control scheme, it is often adequate to assume that the control scheme is maintaining the actual motor speed at the commanded speed. In such cases the cost of the system may be reduced by eliminating the need for a rotor shaft speed sensor. The command frequency may be integrated to determine the flux angle, and the rotor speed may be determined by subtracting the slip frequency from the command frequency.
However, situations exist in which such assumptions are not adequate. This is the case, for example, when a motor drive becomes disconnected from a motor (i.e., the power supply to the motor is interrupted, not necessarily the electrical connection between the motor drive and the motor) and open loop control is no longer present. In this case, with no control present, it is difficult to make any assumptions about the motor parameters.
There are a variety of reasons why a motor drive may become disconnected from a motor. For example, there may be a sudden temporary power loss at the power source that supplies power to the motor and motor drive. Alternatively, it may simply be the case that there are times when it is not necessary to operate the motor, and power is not supplied to the motor during these times.
The fact that the motor drive is disconnected from the motor does not prevent the motor from continuing to rotate. For example, if the motor is used in conjunction with a fan in an air conditioning system, a draft in the air conditioning system may drive the motor at an unknown speed and in an unknown direction. Similarly, if the motor is used in a conveyor system, the force of gravity acting on the motor by way of the conveyed articles and friction may drive the motor at an unknown speed and in an unknown direction.
When a motor drive becomes disconnected from a motor, it eventually becomes necessary to reconnect the motor drive to the motor. To perform the reconnection, it is desirable to determine the above-mentioned parameters, namely, rotor speed, rotor direction, back EMF magnitude and/or back EMF phase angle, before the motor drive is reconnected to the motor. Measuring these parameters is useful because it allows the motor drive to be synchronized to the motor, thereby reducing transients at the moment of reconnection. For example, if the speed of the motor is not determined before reconnection, then the motor drive must assume an initial speed of zero when reconnecting to the motor. This assumption may result in severe transients due to the difference between the frequency of the applied voltage and the frequency of the motor-induced back EMF. The transients are especially severe when the initial motor speed is high and when the motor is rotating in a reverse direction as compared to that commanded by the motor drive. If the current control circuitry or current limiting circuitry of the motor drive is not fast enough, the motor drive can fault due to an overcurrent condition. Additionally, when the motor operates as a generator (i.e., when the frequency of the voltage applied to the motor is less than the motor speed), the DC bus voltage may increase to unacceptable levels and cause damage to the power switches in the motor drive.
It is therefore desirable to determine motor parameters to allow the motor drive to be synchronized to the motor when the motor drive is reconnected and thereby to reduce transients upon reconnection. Additionally, when performing a reconnection, it is desirable to measure these parameters in as little time as possible so that operation may continue as smoothly as possible to make the temporary disconnection as imperceptible as possible.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.